1. Field of the Invention
This invention relates to a method and an apparatus for producing a compound, and more particularly to a method and an apparatus for producing a precursor from a plurality of materials having different vapor pressures, and forming a compound, having a predetermined composition ratio, from the precursor. When materials whose evaporation temperatures are equal to or near a crystallizing temperature of a compound (i.e. they have high vapor pressures) are used, the compound can be crystallized by suppressing evaporation of the materials having high vapor pressures.
2. Description of the Prior Art
Thallium (Tl) oxide superconductors are attracting a great deal of attention as promising superconductors applicable to communication fields such as satellite communications and mobile communications. The thallium oxide superconductor has a relatively small surface resistance Rs, and is relatively free from propagation loss in high frequency ranges such as microwave and millimeter wave ranges. Further, the thallium oxide superconductor has a critical temperature (transition temperature: Tc) of higher than 100 K, and is usable at high temperatures.
Generally, thallium oxide superconductors are TlBaCaCuO superconductors (thallium-barium-calcium-copper-oxide: TBCCO superconductors). In this TlBaCaCuO superconductor, thallium has a high vapor pressure It is well-known that thallium is evaporated before a crystallizing temperature is attained In other words, a composition ratio of thallium is reduced at the crystallizing temperature due to its evaporation, which makes TlBaCaCuO superconductor less reliable.
Because of this disadvantage, the TlBaCaCuO superconductor cannot be formed by the in-situ process which is generally applied to producing a YBaCuO (yttrium-barium-copper-oxide) superconductor. In order to produce the TlBaCaCuO superconductor, a precursor, which contains Tl, Ba, Ca, Cu and O and has an amorphous structure, is formed at a room temperature, and is then thermally treated at a crystallizing temperature of 800.degree. C. In other words, the TlBaCaCuO superconductors are produced in two steps.
The method of producing the foregoing superconductors is prone to the following problems.
(1) At present, the TlBaCaCuO superconductor is only in the research stage, and has not been yet mass-produced. The inventors of the present invention have tried the following crystallizing processes so as to obtain the TlBaCaCuO superconductors. First of all, a precursor containing Tl, Ba, Ca, Cu and O and having an amorphous structure was formed. The precursor was a thin film, and was deposited on a surface of an MgO substrate. The precursor was wrapped with gold (Au) foil, and was placed in an open-type heat treatment furnace. The Au foil was used since it does not react with the precursor, i.e. Tl and O.sub.2 in the precursor, thereby suppressing the evaporation of Tl and O.sub.2 having high vapor pressures. The precursor was thermally treated in the furnace at a crystallizing temperature. Thus, a thin film of the TlBaCaCuo superconductor was obtained.
However, it is very difficult to completely seal the precursor with the Au foil manually wrapped thereon using small pincette. In other words, there are gaps in the Au foil through which evaporated Tl and O.sub.2 may leak. Thus, composition ratios of Tl and O.sub.2 will be reduced in the crystallized TlBaCaCuo superconductor. When Tl is reduced, the TlBaCaCuO superconductor is not superconductive, or has a low critical temperature even when it has superconductive characteristics. Further, when O.sub.2 is reduced, the TlBaCaCuO superconductor has a rough surface, increases a surface resistance, and suffers from poor high frequency characteristics.
(2) Since the Au foil is manually wrapped, clearances through which the evaporated Tl and O.sub.2 leak will have varying sizes. The composition ratios of Tl and O.sub.2 in the crystallized TlBaCaCuO superconductor become variable, which makes it difficult to accomplish the target composition ratios.
(3) In order to improve characteristics of the TlBaCaCuO superconductor, i.e. high frequency characteristics, grain sizes should be larger (i.e. a crystallized surface should be flattened) so as to reduce the surface resistance. Usually, the longer the thermal treatment at the crystallizing temperature, the larger the grain sizes and the smaller the surface resistance. However, the longer the thermal treatment, the more Tl and O.sub.2 will be evaporated and leak, which will worsen the superconductivity and high frequency characteristics.